<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
    结果:<span></span>
</div>
<script>
    /*  let in1 = document.querySelector("#i1");
      let in2 = document.querySelector("#i2");
      let d = document.querySelector("span");
      function f1() {
        d.innerText = in1.value + in2.value;
      }
      function f2() {
        if(isNaN(in1.value) || isNaN(in2.value)){
          d.innerText = "输入错误!"
          return;
        }
        d.innerText = in1.value - in2.value;
      }
      function f3() {
        if(isNaN(in1.value) || isNaN(in2.value)){
          d.innerText = "输入错误!"
          return;
        }
        d.innerText = in1.value * i2.value;
      }
      function f4() {
        if(isNaN(in1.value) || isNaN(in2.value)){
          d.innerText = "输入错误!"
          return;
        }
        d.innerText = i1.value / i2.value;
      }*/
    console.log("代码执行了!");
    function f(x) {
        let i1 = document.querySelector("#i1");
        let i2 = document.querySelector("#i2");
        let s = document.querySelector("span");
        if(isNaN(i1.value) || isNaN(i2.value)){
            alert("输入错误!");
            return;
        }
        switch (x) {
            case 1:
                s.innerText = i1.value * 1 + i2.value * 1;
                break;
            case 2:
                s.innerText = i1.value - i2.value;
                break;
            case 3:
                s.innerText = i1.value * i2.value;
                break;
            case 4:
                s.innerText = i1.value / i2.value;
                break;
        }
    }
</script>
</body>
</html>